1. Field of the Invention
The present invention relates to an optical transmitter which employs a laser diode for emitting light.
2. Description of the Related Art
Laser diodes have their emission efficiency which generally varies with temperature. If a laser diode operates in a narrow temperature range and is associated with an APC (Automatic Power Control) circuit which is capable of correcting a bias current to maintain a desired extinction ratio and waveform duty ratio, then only the APC circuit is sufficient to control the laser diode. However, if the temperature range for a laser diode is too wide for the APC circuit to achieve a desired extinction ratio and waveform duty ratio, then the drive current supplied to the laser diode needs to be corrected depending on the temperature.
Japanese laid-open patent publication No. 7-24055 discloses an optical transmitter having a laser diode whose drive current is controlled by controlling a constant-current source that determines the current of a drive circuit with an APC circuit.
The disclosed optical transmitter, however, is disadvantageous in that when the drive current is varied, the current of the drive circuit has its waveform deteriorated, resulting a deterioration of the waveform of a light signal emitted by the laser diode.
The amount of ringing of the light signal with respect to the drive current is generally constant irrespective of the magnitude of the drive current. The conventional optical transmitter has suffered a problem in that when the ambient temperature around the laser diode drops thereby to lower the drive current supplied to the laser diode, the proportion of ringing in the waveform of the light signal increases.